This invention relates generally to the field of magnetic refrigeration and to active magnetic regenerative refrigeration apparatus.
Magnetic refrigeration, using magnetocaloric materials as a working element, promises to address important limitations associated with conventional refrigeration technologies which use gas compression and expansion. Magnetocaloric materials have the property that their temperature changes when a magnetic field is applied. In the case of a material near a transition from a ferromagnetic state to a paramagnetic state, the material will warm when magnetized and cool when demagnetized. Magnetic refrigeration can avoid the use of volatile fluids, such as chlorofluorocarbons (CFC""s), that may harm the environment. Magnetic refrigeration can be more energy efficient than conventional refrigeration technologies. Magnetic refrigeration can also produce very low temperatures, which can enable, for example, cost-effective production of liquid hydrogen for use as an alternative fuel for transportation and other applications. Thus, there has long been motivation to find an effective apparatus for magnetic refrigeration.
Many magnetic refrigerators use active magnetic regeneration as an operating principle. The term active means that a magnetic field is applied to a magnetocaloric material and then removed. A regenerator is a thermal device that transfers heat into a heat transfer medium during one stage of a regenerative cycle, and then transfers heat out of that heat transfer medium during an opposite phase of the regenerative cycle. Active magnetic regeneration refers to a regenerator which applies a time-varying magnetic field and reciprocating flow of a heat transfer medium to an elongated container of magnetocaloric materials, to produce a temperature gradient along the container of magnetocaloric materials and to enable heat transfer into and out of the heat transfer medium. Active magnetic regeneration may be used in a magnetic refrigerator, to provide cooling, or in a heat pump, to provide heating.
In a typical active magnetic regenerator device, a bed of magnetocaloric material which is porous to a heat transfer fluid is connected to two heat exchangers, with mechanisms provided for magnetizing and demagnetizing the bed, and for effecting reciprocating fluid flow through the bed of magnetocaloric material from one heat exchanger to the other. A typical active magnetic regenerator device usually performs four basic operations: (1) bed magnetization, which increases the temperature of the magneto-caloric material in the bed by the magnetocaloric effect; (2) fluid transfer in the cold side to hot side direction, with warmed fluid flowing out of the bed into a hot side heat exchanger, where heat can be released; (3) bed demagnetization, which reduces the temperature of the magnetocaloric material in the bed by the magnetocaloric effect; and (4) fluid transfer in the hot side to cold side direction, with cooled fluid flowing out of the bed and into a cold side heat exchanger, where heat can be absorbed.
In accordance with the present invention, a magnetic refrigeration apparatus has a magnetic regenerator bed containing magnetocaloric material, a magnet, and a means for moving the magnet in a path adjacent to the magnetic regenerator bed, whereby the motion of the magnet produces a variation of magnetic field strength in the magnetic regenerator bed, which in turn leads to a variation in temperature of the magnetocaloric material.
In one aspect of the invention, the magnetic regenerator bed is comprised of a plurality of compartments arranged in a ring, and a magnet is attached to a rotating assembly which moves the magnet in a path around the ring.
In another aspect of the invention, a method of transferring heat comprises rotating a magnet around an annular container which includes a magnetic regenerator compartment containing magnetocaloric material. The rotation of the magnet produces a cyclic variation in magnetic field strength in the magnetic regenerator compartment. The variation in magnetic field strength causes a cyclic variation in temperature of the magnetocaloric material in the magnetic regenerator compartment.
In another aspect of the invention, a heat transfer apparatus comprises an annular container which includes a plurality of magnetic regenerator compartments, a magnet mounted for rotation about the central axis of the annular container, a heat exchanger, and a valve. Each magnetic regenerator compartment has a hot side and a cold side, and each magnetic regenerator compartment contains magnetocaloric material that allows the flow of heat transfer fluid through such magnetocaloric material. The valve has an axial port and a plurality of radial ports. The axial port of the valve is connected by a pipe to the hot heat exchanger, and each radial port of the valve is connected by a pipe to the hot side of a magnetic regenerator compartment. The rotation of the magnet produces a cyclic variation in magnetic field strength in the magnetic regenerator compartments. The variation in magnetic field strength causes a cyclic variation in temperature of the magnetocaloric material in the magnetic regenerator compartments. The valve is used to cause the heat transfer fluid to flow from the magnetic regenerator compartments to and from the heat exchanger at the appropriate time to exploit the cyclic variation of temperature of a magnetocaloric material for heat transfer.
In another aspect of the invention, a heat transfer apparatus includes an annular arrangement of a plurality of magnetic regenerator compartments containing magnetocaloric material, a magnet mounted for rotation about the central axis of the annular arrangement of magnetic regenerator compartments, and a valve having an axial port, a rotating inner assembly, and a plurality of radial ports, wherein the rotating inner assembly rotates synchronously with the rotation of the magnet to connect heat transfer fluid flow between the axial port and one or more of the radial ports.
A rotating magnet magnetic refrigerator according to such a preferred embodiment of the invention has several desirable features. Work input to the device is via circular motion that may be at constant speed. Forces are well balanced, so that the net drive force is mainly that necessary to drive the refrigeration process, and this force is nearly constant. Reciprocating flow occurs to the magnetocaloric material, allowing regenerative cycles to be performed, and yet dead volume effects in the heat exchangers or between the magnetocaloric material and the heat exchangers are minimized. Finally, the seals used in the valve can be of simple design, are exposed to minimal wear, and generate minimal friction.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.